Mosaic electrode structure



Aprili. 23 M. BANDRINGA ET AL,

MOSAIC ELECTRODE STRUCTURE Filed Feb An a INVENTORS MENSE BANDRINGA AND MARTEN CORNELIS TEVES ATTORNEY.

Patented Apr. 23, 19 W uulrsp stares MOSAIC suac'raomz s'rnuc'ruan Menze Bandringa and Marten Cornelis Teves. Eindhoven, Netherlands, assignors, by mesne assignments, to Radio Corporation oi America, I New York, N. Y., a corporation of Delaware Application Fcbruary'zli, 193s, Serial No. 193,129 In Germany March 5, 1937 4 Claims.

.Our invention relates to improvements in cathode ray television transmitting tubes and particularly to an improved photosensitive electrode structure for use in such tubes.

r The mosaic electrode of television transmitting tubes of the type known as the iconoscope frequently comprises a thin sheet of insulating material such as a sheet of mica which constitutes the dielectric of a multiple condenser. The mica sheet is coated on one side with a conductive layer, such as a sheetof metal, which serves as a common electrode of the condenser. .On the other side of the mica sheet there are, distributed over the entire surface, mutually separated con.

ductive particles which are electrically insulated from each other and which are coated or sensitized with .a-substance capable of emitting photoelectrons or secondary electrons. These sensitized particles constitute the complementary electrodes of the multiple condenser and are referred to hereinafter as sensitive particlesfif They may jointly cover, about one quarter toone'half of the surface of the dielectric. Thus, the voids left between the sensitive parts occupy about one half to three quarters of the surface.

When an optical image is projected on the mosaic electrode the sensitive particles acquire electrostatic charges which vary in magnitude in accordance with the incident light, but when the electrode is scanned bythe cathode ray beam to neutralize the charges, it is found that electrons find their way through the voids between the sensitive particles and dislodge from the exposed dielectric secondary electrons which influence the electrostatic charges on the sensitiveparticles in an unfavorable way thus introducing spurious slgnalswhich interfere with the desired operation of the device.

An object of our invention is to provide an improved mosaic electrode for use in television transmitting tubes wherein secondary electron emission from the exposed dielectric between the sensitive particles is suppressed Another object of our invention is to provide a mosaic electrode which is uniformly responsive to the influence of light over its active surface.

In accordance with our invention, the sheet of insulating material of dielectric has-applied to it, before the formation of the sensitive particles, a braking layer which is formed from metal only slightly capable of emitting secondary electrons and which prevents the electrons of the cathode ray beam which enter the voids between the sensitive particles from dislo ging secondary electrons from the dielectric. This ensures the advantage that the electrostatic charges on the sensitive particlesare not influenced by secondary electrons or are infiuencedthereby to a much less extent than previously, so that the charges on the sensitive particles correspond more satisiactorily to the degrees of light and shade of the optical image focused thereon.

These and other objects, features and advantages of our invention will appear from the following description taken in connection with the accompanying figure which shows'a greatly enlarged sectional view of a mosaic electrode made in accordance with our invention. a

As best shown in the figure our new and improved mosaic electrode comprises a foundation sheetof insulating material such as the sheet of mica ill with a uniform coating or. film of an electrical conductor such as the metal film ii on one side to serve as the signal plate ofthe electrode,'and a multiplicity of individually separated photosensitive particles I? supported from the opposite side of the sheet of mica ill by an intermediate film or layer of metal is which will hereinafter be termed the braking layer. The sensitive particles l2 are mutually, insulated one from the other except for the very-slight electrical conduction which may take place through the braking layer.

The braking layer must be thick enough to obtain the desired advantages, and must also be of such a kind that it does not electrically conmeet the sensitive particles or at least connects them by a very high resistance, so that the conductivity in the layer does not exceed that which is admissible for the operation oi the tube in which the mosaic is used. Such a layer of sufficiently high resistance maybe obtained by male ing the layer very thin. The resistance of the braking layer over a width oi looms. is preferably at least 10 ohms per cm, of length, as such a layer, has negligible electrical conductivity at the voltages applied to the film during operation.

It is advantageous to use for the braking layer metals which do not oxidize in air or on which only a very thin oxide film is formed so as not to increase the secondary emission of the braking layer. Any oxide film that may be present should, preferably, be less than 20 atoms thick.

Suitable metals for the braking layer are, for example, aluminum, platinum, gold, and chromium. Aluminum layers have a particular advantage in'that they do not have appreciable conductivity even when of comparatively large mean-thickness, for example, a mean thickness of approximately 100 atoms. The thicker the braking layer, the greater is the protection against penetration of primary electrons into the insulating material.

The coating of the mica with athin film of metal which serves as the braking layer may be done most conveniently by mounting the mica sheet within a sealed envelope together with a tungsten filament which may be heated to incandescence and which has been coated with the aluminum or other metal with whichthe mica sheet is to be coated. The envelope is then evacuated and the filament heated to flash of? enough of the metal with which it is coated to produce the desired film on the mica sheet. For example, good results have been obtained with a a" x sheet of mica by placing the filament approximately 10 inchesfrom the surface of the mica. When using aluminum as a metal with which to coat the mica, about 42 mgs. of aluminum is vaporized from. the filament to give a deposit of 0.67 mg. on the mica sheet which corresponds to a layer 'of aluminum approximately 100 atoms thick. The exact quantity of aluminum to be evaporated may be applied to the tungsten filament which is then heated to a temperature of 1,000 to 1200 C. for a period of approximately 10 minutes to insure vaporization of the entire amount.

The above data are calculated from the atomic weight oialuminum and the crystalline structure of the metal as determined by X-ray analysis. The weight of metal to be evaporated for any thickness of film on the mica. sheet and. for any "lirakingla'yer to a less extent than on the mica sheet, so that the particles are less apt to flow together and come into electrical contact with each other.

For this reason it is advantageous to first form the filmof metal i3 or braking layer over all the surface of the mica sheet i0, and then apply to the, exposed surface of this layer the individually separated metal particles itwhich are subsequently oxidized and sensitized with caesium or other photosensitive material during theevacuation of the tube. A suitable process for forming, oxidizing, and pliotosensitizing the. metal particles is disclosed by S. F. Essie, in his U. 8. Patent 2,065,570. f 7

While we do not wish to be limited by any particular theory as to the operation oi our new and improved mosaic electrode when used in television transmitting tubes, it is believed that the braking layer which is intermediate the dielectric and the sensitive particles reduces the secondary electron emission liberated 'by'the action of the scanning beam when passing over the voids between the sensitive particles. It has been found that in the use of tubes incorporating our new and improved mosaic electrodethe reproduced picture corresponds well in light and shade detail to the transmitted optical image and is lacking in shading effects which may be attributed to high secondary electron emission from the areas of the dielectric between the sensitive particles.

awhile we have indicated the preferred embodiments of our invention of which we are now aware and have'also indicated only one specific application for which our invention may be .em ployed it will be apparent that our invention is by no'means limited to the exact forms illustrated,

or the use indicated, but that many variations may be made in the particular structure used and assess? ing of metal on one side of said sheet of mice. to serve as a signalplate for the electrode, a film of metal selected from the group of metals consisting of aluminum, platinum, gold and chromium on the other side of said sheet of mica'having an electrical resistance over a width of 10 centimeters of at least 10 ohms per centimeter of length, and a multiplicity of mutually separated photosensitive particles on said film coextensive with ing material, a coating of aluminum oxide less" than 20 molecules thick on said aluminum film to-suppress the emission of secondary electrons originating in areas of said coating of aluminum oxide when subjected to high velocity electron bombardment, a mosaic of mutually separated metal particles on said coating of aluminum.

oxide leaving'the said areas between the particles which may. be subjected to electron bombardment and a film of electrically conducting material on the opposite side of said sheet of insulating material from said film of aluminum and coextensive with said mosaic of mutually sepa-' rated metal particles to form a signal plate for the electrode.

3. A mosaic electrode for a television transmitting tube comprising a sheet of insulating material, a coating of electrically conductive material on the surface of one side of said sheet of insulating. material to serve as a signal plate for the electrode, a film having, high electrical re-,.

secondary electrons when said film is bombarded by high velocity electrons. said film of metal having negligible electrical conductivity, and a multiplicity of mutually separated photosensitive 'metal particles'on said film coextensive with said coating of electrically conductive material, the

areas of the film ofmetal between the particles hundred atoms thick on one side of saidsheet of insulating material to suppress secondary electron emission when bombarded 'by high velocity electrons, a mosaic of mutually separated metal particles incompletely covering said film, leaving the film of aluminum between the particles exposed sothat when said film is bombarded by high veiocity electrons. the emission of secondary electrons originating between the said particles Willi-be suppressed, and afilm ofelectrically conductive material on the opposite side of said sheet {of insulating material coextensive with said mosaic oi mutually separated metal particles.

M'ENZE EANDRINGA. IWARTEN CORNELIS TEVES. 

